During automatic casting operations a crucible is frequently used to transport molten metal from a holding or refining furnace to the molds. Although an effort may be made to melt and transport molten metal that contains a minimum of impurities, the molten metal transported by a crucible to a mold will usually contain impurities which are detrimental to the resulting cast product. These impurities may include oxides of the metal, inclusions introduced by fluxes used during melting, or products formed by interaction of the molten metal with the furnace lining.
Techniques for removing impurities from the molten metal include filtering the molten metal in the holding or melting furnace, and/or placing a gate filter in the mold to filter the molten metal as it enters the mold. If the molten metal is filtered in the holding or melting furnace impurities may be picked up by the molten metal during transfer from the furnace to the crucible, and from the crucible to the mold. If a filter is placed in a mold gate excess superheat may be required, the time to fill the mold may be excessive, and in addition mold gate filtering is costly. Excess superheat can increase gas solubility in the molten metal and will increase the energy costs for the melting and casting operation.